The only thing to do is make her feel secure in any way possible. She's young enough that the feeling of not having enough will probably not stick with her once she's had love and security for a while.
I suggest doing something really fun (go to the park, play with her, color...whatever 2 year olds do.

) before she sits down to eat. Maybe let her help cook...can 2 year olds do that? Is 2 too young to stir something in a bowl or whatever?
And teaching her proper nutrition is a good idea. But she'll learn eventually from observing her foster family's eating habits. It'll click with her, I promise. They'll just have to be patient and give her lots of love, which I'm sure they're doing.